Implantable medical devices (IMDs) are available for monitoring and treating patients for a variety of conditions. Such devices often include physiological sensors for sensing signals correlated to physiological events or conditions. The detection or monitoring of physiological events or conditions is used for diagnosing a patient condition, determining a need for therapy or medical intervention, and sometimes for controlling a therapy delivered automatically by the implantable device.
Tissue oxygen saturation (O2Sat) is one variable that can be monitored to assess a patient condition. Optical sensors are available or have been proposed for monitoring O2Sat (sometimes referred to as StO2) or an index of O2Sat. A number of factors can influence O2Sat measured at a body tissue site. Lung function, cardiac function, consumption of oxygen at the tissue site, and other factors can influence O2Sat measurements. As such, other monitored physiological signals may be needed in conjunction tissue O2Sat, along with clinical patient examination, in order to infer a patient condition.